The invention relates to a device for the continuous treatment of the surfaces of thin sheet material, hereinafter referred to as a web by means of an electrical corona discharge. In such devices, a treatment roller for guiding the web is arranged in a partially evacuated treatment space. The web is closely guided into the treatment space between one pair of rollers and out of the treatment space between another pair of rollers. At least one corona electrode is positioned opposite the treatment roller and is composed of insulation and a high voltage element. Such a device is described in German Publication DE-OS No. 31 34 615.
Recently, it has been determined that the corona treatment in a slightly evacuated treatment space entails great advantages as compared to the use of normal atmospheric pressure. In that case, the corona develops at low voltage, so that the energy consumption and the formation of ozone are reduced. The latter is significant with respect to protection of the environment.
The lower voltage level results in the fact that the web leaving the device is electrostatically charged to a lesser extent. This leads to improved adhesion and wetting characteristics for the web, which is a great advantage for webs which serve as photographic carrier material.
In prior corona treating chambers in which the treatment area is to be evacuated, it is necessary to provide a seal around the guide rollers which feed the web into the chamber. The seal is typically a concave member conforming to the guide rollers which is located close enough to the guide rollers to restrict the entrance of air into the chamber. This type of seal is generally satisfactory for slight pressure reductions, but is incapable of supporting higher levels of vacuum inside the chamber.
The device according to the above mentioned German Publication DE-OS No. 31 34 615 features a corona electrode inside the treatment chamber. When high voltage is applied, the corona electrode not only discharges in the direction of the treatment roller, but also in other directions, especially toward the walls of the treatment chamber. This is especially true for the high voltage wire which must be led into the treatment chamber to supply the corona electrode. This discharge into non-desired directions leads to energy loss and reduces the quality of the treatment on the web.
Aside from this, the previously known device is constructed in a relatively costly fashion, since it not only has discharge electrodes within the treatment chamber, but the feed and exit of the web is accomplished by means of two pairs of rollers through an antechamber and additionally two guide rollers underneath the treatment roller.